Mode-locked ultra-short pulse fiber laser has many applications, such as micro machining, laser surgical operation, optical frequency measurement, etc. Nonlinear polarization rotation (NPR) has been used as an artificial saturable absorber in a fiber ring laser to start the mode-locking for many years. Traditionally, NPR mode-locked fiber ring laser requires the polarization of the laser to be elliptical at the input end of the fiber and nonlinear phase generated in the fiber makes the polarization ellipse rotate. To allow the polarization ellipse to rotate along the fiber, usually non-polarization-maintaining (non-PM) single mode fiber is used. The refraction index of non-PM single mode fiber can be changed by environmental temperature or pressure variation, thus, the polarization state of the laser light in the laser resonator can be changed and mode-locking may fail. Therefore, an environmental stable mode-locked fiber laser is urgent for reliably operating the ultra-short pulse fiber laser.